Bonded heat exchanger matrix and corresponding bonding method

ABSTRACT

A heat exchanger metal matrix has a stack of components in which at least one of the components is bound by an adhesive layer based on an epoxide resin containing a corrosion inhibitor, and the adhesive layer is loaded with 20 to 60% by mass of a heat conductor so that a heat conductivity of the adhesive layer is in a range of 2 to 5 W/m/K. The heat exchanger metal matrix may be applied to corrosive environments, notably marine environments.

The invention relates to the field of metal heat exchangers, notably in aluminium, of the type with etched plates, of the type with separating metal sheets, bars and fins or including a combination of both of these types.

These heat exchangers are currently used in methods for separating gases from air and for liquefying natural gas, because of their very good energy performance properties, of very low temperature mechanical strength and lightness.

In a known way, the matrix of these heat exchangers is assembled by brazing and their fluid dispensing heads are welded on the brazed matrix.

The thereby formed heat exchangers are of a purely metal nature and sensitive to corrosion. Their field of application is therefore limited to clean and not very corrosive environments. Notably, they support neither sea water nor a marine atmosphere.

The origin of this incompatibility stems from diffusion phenomena involved at the interface of the constitutive components of the exchanger and of the brazing, which induce metallurgical modifications of the initial material. After cooling, the presence of intermetallic precipitates is assumed to be one of the major causes of the occurrence of corrosion pits, following the formation of electrochemical cells favoring etching of the adjacent metal base.

Anticorrosion coatings exist, but their application on these types of equipment remains a problem. The anticorrosion coating may either be applied on the individual components of the matrix before the assembling and brazing step, or on the finished matrix after brazing.

The first method has the disadvantage of only being able of using anticorrosion coatings remaining stable at brazing temperatures and not perturbing the brazing. The second method does not give the possibility of uniformly depositing the anticorrosion coating and in the whole of the brazed matrix since the latter includes many crevices with access difficulties.

An object of the invention is therefore to make a heat exchanger metal matrix which better resists to corrosion while remaining solid and a good heat conductor. Such a matrix should notably be adapted to marine applications.

According to the invention, this object is achieved by a heat exchanger metal matrix, characterized by a stack of components, notably of etched plates or of fins, separating metal sheets and bars, or a combination of both types of stacks, wherein at least one portion of said components are bound together by a layer, preferably with a thickness comprised between 20 and 150 μm, of a structural adhesive based on epoxy resin containing a corrosion inhibitor and loaded with 20 to 60% by mass of a heat conductor ensuring a heat conductivity of the adhesive from 2 to 5 W/m/K.

By binding at least one portion of the components of the matrix by means of said adhesive, it is possible to do without the brazing and the traditional filler metal which is sensitive to corrosion. By the selected adhesive formulation, the matrix is protected against corrosion and retains its mechanical and thermal performances.

The matrix according to the invention finds a particularly advantageous application in heat exchangers placed in a corrosive environment, notably in a marine medium, whether the heat exchangers are immersed in water or in a marine atmosphere.

According to preferred embodiments, the matrix according to the invention comprises one, several or all of the following features, in any technically possible combinations:

-   -   the heat conductor of the adhesive is based on metal and/or         ceramic;     -   the corrosion inhibitor of the adhesive is based on zinc oxides;     -   the components are coated with an adhesive holder, notably a         conversion layer and/or a layer of adhesive holding primer;     -   the conversion layer has a thickness comprised between 1 and 50         μm and preferably comprised between 5 and 20 μm;     -   the components are in aluminium or an aluminium alloy, and the         conversion layer is in alumina;     -   a portion of the components are brazed together.

The present invention also relates to a heat exchanger including a matrix as defined above, and preferably at least one head for dispensing fluid adhered to the matrix, notably with said adhesive.

Another object of the invention is to achieve a method for assembling a heat exchanger metal matrix adapted to corrosive environments. According to the invention, this object is achieved by a method for assembling a heat exchanger metal matrix, characterized by the steps:

-   -   a) providing the components of the matrix;     -   b) depositing a structural adhesive based on epoxy resin         containing a corrosion inhibitor and loaded with 20 to 60% by         mass of a heat conductor ensuring a heat conductivity of the         adhesive from 2 to 5 W/m/K on at least one portion of the         components;     -   c) stacking the components so as to obtain a stack; and     -   d) ovening the stack so as to harden said adhesive and to         thereby obtain the matrix.

According to preferred embodiments, the method according to the invention comprises one, several or all of the following features, in any technically possible combinations:

-   -   the step consisting of applying an adhesive holder on the         components before step b);     -   the adhesive holder application comprises a first step of         anodization or phosphorization, and/or a second step of applying         a holding primer by dipping the component in the primer or         projecting the primer on the component;     -   the step consisting of drying and heating the components covered         with holding primer at a temperature comprised between 50 and         200° C. for a period comprised between 30 and 120 mins, so as to         bind the holding primer to the component.     -   step b) comprises:         -   i) providing the adhesive as a paste and spreading it out on             the component by means of a doctor blade, or         -   ii) co-lamination of the adhesive on the component;     -   step d) comprises a first phase for maintaining the stack at a         temperature comprised between 50 and 120° C. for a minimum         period of thirty minutes followed by a second phase for         maintaining the stack at a temperature comprised between 150 and         250° C. for a minimum duration of one hour.     -   step d) comprises the phase for maintaining the stack under         compression at a pressure above 100 kPa.

The invention consists, except for the arrangements discussed above, in a certain number of other arrangements which will be more explicitly discussed hereafter as regards exemplary embodiments described with reference to the appended drawings, but which are by no means limiting. Among the drawings:

FIG. 1 is a perspective and exploded view illustration of a matrix during stacking according to an exemplary embodiment of the invention;

FIG. 2 is a detail 7 of the matrix of FIG. 1 showing the adhesive bonding of the components of the matrix;

FIGS. 3 to 6 illustrate the treatment of a separating metal sheet of the matrix of FIG. 1 according to the assembling method of the invention; and

FIGS. 7 to 9 illustrate the treatment of a fin of the matrix of FIG. 1 according to the assembling method of the invention.

Subsequently, in order to simplify the description of the invention, reference will be made to a heat exchanger matrix with separating metal sheets, bars and fins, being aware that the invention also applies to an exchanger with etched plates, or to an exchanger comprising a combination of separating metal sheets, bars and fins and etched plates. Further, an aluminium matrix will be described subsequently. Nevertheless, the invention also covers matrices consisting of other metals, such as notably steel.

With reference to FIG. 1, a stack of a matrix 2 being made may be schematically seen as illustrated. In a known way, the matrix 2 consists of a stack 3 of components, i.e. fins 4, separating metal sheet 5, and aluminium bars 6.

The particularity of the matrix 2 is visible in FIG. 2. An enlarged illustration of the area 7 of the matrix 2 indicated in FIG. 1 is distinguished therein. A fin 4 is located between two separating metal sheets 5 and bound to the latter. Both separating metal sheets 5 have two opposite faces 8 and 9, and the fin 4 has two opposite faces 10 and 11.

According to the invention, the separating metal sheets 5 and the fin 4 are covered on their two opposite faces 8, 9 and 10, 11 with an adhesive holder 12. The adhesive holder 12 consists of two layers, i.e. a conversion layer 13 extending over the faces 8, 9, 10, 11, and an adhesive holding primer layer 14 deposited on the conversion layer 13. The conversion layer 13 consists of alumina. The primer layer 14 consists of a resin from the family of epoxide resins in which are integrated corrosion inhibitors, for example zinc salts. The conversion layer 13 has a thickness I comprised between 1 and 50 μm and preferably comprised between 5 and 20 μm. The primer layer 14 preferably has a thickness d of a few micrometers.

An adhesive layer 15 deposited on both opposite faces 8, 9 of the separating metal sheets 5 ensures the connection between the separating metal sheets 5 and the fin 4. Preferably, the thickness e of the adhesive layer 15 is comprised between 20 and 100 μm.

The adhesive 15 is a structural adhesive from the family of epoxide resins. The adhesive 15 contains corrosion inhibiting elements, for example zinc salts or oxides. The adhesive 15 is also loaded with 20 to 60% by mass of additional elements which substantially increase its heat conductivity, for example of metal or ceramic origin. Thus, the heat conductivity of the adhesive 15 is located between 2 and 5 W/m/K.

The method for assembling the matrix 2 will now be described, with reference to FIGS. 3 to 9.

In a first step, the separating metal sheets 5 are made in aluminium, an example of which is shown in FIG. 3, the fins 4, an example of which is shown in FIG. 7, and the bars 6 of the matrix 2.

In a second step, the opposite faces 8, 9 of the separating metal sheets 5, the opposite faces 10, 11 of the fins 4, as well as the bars 6 are anodized in order to grow conversion layers 13 in alumina (Al₂O₃). The anodization is preferably sulfuric or chromic anodization. The result is illustrated in FIGS. 4 and 8.

If the matrix 2 is assembled from steel components, the anodization will then be replaced with a phosphatization operation.

In a third step, the conversion layers 13 are covered with the holding primer layers 14. Preferably, this step is carried out by dipping the bars 6, the fins 4 and the separating metal sheets 5 in an aqueous solution of the holding primer. Thus, the components 4, 5, 6 are coated with holding primer 14. In an alternative, the holding primer 14 is applied on the components 4, 5, 6 by projection.

It will be ensured that the application of the holding primer 14 is accomplished in a homogenous way on the whole of the surfaces, in order to subsequently guarantee good adherence of the whole of the components 4, 5, 6. The result of the third step is illustrated in FIGS. 5 and 9.

The application of the holding primer 14 is followed by drying punctuated by heating in order to chemically bind the holding primer 14 to the treated surfaces. The connection between the holding primer 14 and the anodized surfaces 13 is preferably obtained by a hot air treatment carried out at a temperature comprised between 50 and 200° C., this for a period which preferably ranges between 30 and 120 mins. In a particularly preferred way, the anodized components 4, 5, 6 coated with the holding primer 14 are maintained at about 90° C. for about 120 mins.

In a fourth step, the adhesive 15 is only applied on the holding primer 14 of the separating metal sheets 5. This may be made as an adhesive paste uniformly deposited in layers by means of a doctor blade in order to end up with a sufficient and uniform thickness, or else by applying a film which will be co-laminated on the separating metal sheets 5, or by any other means giving the possibility of providing the deposit of adhesive 15 on the separating metal sheets 5. The application of the adhesive 15 should observe as much as possible a residual thickness of about 20 to 150 microns in order to both ensure the role of a binder and the role for protecting the underlying separating metal sheet 5. The result of the fourth step is illustrated in FIG. 6.

The actual fact of being able to carry out the steps two to four on the individual components 4, 5, 6, the surface of which is easily accessible, is a clear advantage. The control of predetermined parameters like the thickness or the uniformity of the deposit is facilitated by observing this methodology. The method according to the invention is thus advantageously distinguished from methods in which the preparation of the surfaces of the components 4, 5, 6 is accomplished a posteriori after assembling the stack 3.

In a fifth step, the components 4, 5, 6 are stacked in order to obtain the stack 3. The sixth step consists of an ovening phase at a temperature below 150° C. of the stack 3 in order to cure (polymerize) the adhesive 15. At the end of the ovening, a solid and corrosion-resistant matrix 2 is obtained. The ovening for example consists in heating and maintaining the stack 3 at 90° C. for four hours, followed by heating and maintaining the stack 3 at 120° C. for one hour. This may be carried out in a press-oven, in an oven with forced convection or any other equivalent heating method. A device for clamping the stack 3 is preferably used in order to optimize the connection of the components 4, 5, 6 during the polymerization process. The clamping device may for example maintain the components 4, 5, 6 under a constant load exceeding 100 kPa.

The completed matrix 2 may then be provided with heads for dispensing fluid in order to form a heat exchanger. The fluid dispensing heads may be directly adhesively bonded on the surface of the matrix 2 with said adhesive 15.

Alternatively, the fluid dispensing heads are welded to the matrix 2 via intermediate parts nested beforehand into the matrix 2 during its stacking according to a male/female configuration. Said intermediate parts give the possibility of moving the welding area sufficiently away from the matrix 2 in order to avoid degradation of the adhesive joints of the matrix 2 by the high temperatures prevailing during welding. In this case, the seal of the connection between the intermediate part and the matrix 2 is ensured by an elastomer based on silicone.

By means of the assembling method according to the invention, each metal component 4, 5, 6 is covered with multiple layers which act as barriers to diffusion and to propagation of corrosion sources.

According to an alternative embodiment of the invention, certain components 4, 5, 6 of the matrix 2 are brazed and others adhesively bonded. For example, fluid passages of the matrix 2 intended to receive the corrosive fluid such as sea water are delimited by adhesively bonded components 4, 5, 6, while the fluid passages of the matrix 2 intended for fluids, for which the pressure of use is outside the field of use of the adhesive 15, for example ammonia, are delimited by brazed components 4, 5, 6.

In order to obtain this mixed assembly, in a first phase, the components 4, 5, 6 having to be brazed are brazed according to the usual method for manufacturing a brazed heat exchanger. Sub-assemblies of the matrix 2 are made with the whole of these components 4, 5, 6, being aware that the brazing is only present on the surfaces which have to be brazed. After brazing, the brazed sub-assemblies and the remaining components 4, 5, 6 are coated with adhesive 15 and stacked for forming the stack 3. The stack 3 then undergoes ovening described above (step six). The low temperature of the ovening gives the possibility of not degrading the brazing carried out in a first phase.

According to another alternative embodiment of the invention, an adhesively bonded/brazed mixed matrix is assembled by using low temperature brazing (melting temperature of the brazing below 200° C.). This gives the possibility of first assembling the whole stack 3 with its sub-assemblies coated with adhesive and brazing, and then ovening said stack 3 so as to thereby cure the adhesive and at the same time merge the brazing.

By means of the adhesive bonding according to the invention, the heat exchanger matrix proposed may be applied in corrosive environments while retaining the required heat performance and pressure resistance properties. Further, the method according to the invention gives the possibility of assembling heat exchanger matrices of a large volume. 

1. A heat exchanger metal matrix comprising: a stack of components wherein at least one said components is bound by an adhesive layer based on an epoxide resin containing a corrosion inhibitor, and the adhesive layer is loaded with 20 to 60% by mass of a heat conductor so that a heat conductivity of the adhesive layer is in a range of 2 to 5 W/m/K.
 2. The heat exchanger metal matrix according to claim 1, the heat conductor of the adhesive layer is based on a metal and/or a ceramic.
 3. The heat exchanger metal matrix according to claim 1, the corrosion inhibitor of the adhesive layer is based on zinc oxides.
 4. The heat exchanger metal matrix according to claim 1, wherein the components are coated with an adhesive holder.
 5. The heat exchanger metal matrix according to claim 4, wherein the adhesive holder comprises a conversion layer having a thickness between 1 and 50 μm.
 6. The heat exchanger metal matrix according to claim 5, wherein the components are made of an aluminium or an aluminium alloy, and the conversion layer is made of an alumina.
 7. The matrix according to claim 1, wherein a part of the components is brazed.
 8. A heat exchanger comprising the matrix according to claim
 1. 9. A method for assembling a heat exchanger metal matrix comprising: a) providing components of the heat exchanger metal matrix; b) applying a structural adhesive (15) based on an epoxide resin containing a corrosion inhibitor and the structural adhesive is loaded with 20 to 60% by mass of a heat conductor so that a heat conductivity of the structural adhesive is in a range of 2 to 5 W/m/K on at least a part of the components; c) stacking the components so as to obtain a stack; and d) ovening the stack as to cure said structural adhesive and thereby obtain the heat exchanger metal matrix.
 10. The method according to claim 9, further comprising: applying an adhesive holder on the components before applying the structural adhesive.
 11. The method according to claim 10, wherein b) applying the adhesive holder comprises: anodizing or phosphorizing; and/or applying a holding primer by dipping the components in a primer or projecting a primer on the components.
 12. The method according to claim 11, further comprising: drying and heating the components covered with the holding primer at a temperature between 50 and 200° C. for a period between 30 and 120 mins so as to bind the holding primer to the components.
 13. The method according to claim 9, wherein b) applying the adhesive holder comprises: i) providing the structural adhesive as a paste and spreading the paste of the structural adhesive out on the components by a doctor blade, or ii) co-laminating the structural adhesive on the components.
 14. The method according to claim 9, wherein d) ovening the stack comprises: maintaining the stack at a temperature between 50 and 120° C. for a minimum period of 30 minutes followed by maintaining the stack at a temperature between 150 and 250° C. for a minimum period of one hour.
 15. The method according to claim 9, wherein d) ovening the stack comprises: maintaining the stack under compression at a pressure above 100 kPa.
 16. The heat exchanger metal matrix according to claim 1, wherein the stack of the components is a stack of etched plates or fins, separating metal sheets and bars, or a combination of both types of stacking.
 17. The heat exchanger metal matrix according to claim 1, wherein the adhesive layer has a thickness between 20 and 150 μm.
 18. The heat exchanger metal matrix according to claim 4, wherein the adhesive holder is a conversion layer and/or a layer of an adhesive holding primer.
 19. The heat exchanger metal matrix according to claim 5, wherein the thickness of the conversion layer is between 5 and 20 μm.
 20. The heat exchanger according to claim 8, further comprising: at least one fluid-dispensing head adhered to the matrix with said adhesive layer. 